Conventional techniques for carrying out industrial-scale crystallization of sodium carbonate monohydrate involve evaporative crystallization and the unaltered boiling temperature of a solution of sodium carbonate at the barometric pressure of the plant location, which is dependent on the geographic elevation of the plant.
However, conventional techniques suffer from problems such as poor crystal quality due to impurities present in the feed liquor. As an example, sodium chloride lowers the transition temperature of sodium carbonate monohydrate to anhydrous sodium carbonate. Crystallization of anhydrous sodium carbonate is undesirable because of the negative slope of its temperature-solubility curve, which results in rapid fouling of heat transfer equipment and frequent shutdowns to clean anhydrous sodium carbonate from surfaces of the heat transfer equipment. As such, there is an ongoing need for improving this step to avoid or diminish the aforementioned problems, especially in view of the large scale (e.g., about 100 metric tons (MT) per hour) under which the step is typically carried out.
The present invention addresses the shortcomings associated with conventional technologies by implementing changes in the operating conditions of the sodium carbonate monohydrate crystallizers to substantially reduce or eliminate the potential for precipitation of anhydrous sodium carbonate on heat transfer surfaces. Typically, these changes result in the lowering of the operating pressures and boiling temperatures of the sodium carbonate monohydrate crystallizers to significantly improve plant onstream time and overall plant yield. By employing these changes, it was unexpectedly observed that higher concentrations of impurities, such as sodium chloride, may be tolerated without causing the undesirable transition of sodium carbonate monohydrate to anhydrous sodium carbonate. This methodology also allows for higher impurity concentrations during the crystallization step, thereby desirably minimizing the impurity purge stream volume.
Soda ash (sodium carbonate (Na2CO3)) is presently produced on a commercial scale by three industrial processes: the trona ore process (which uses natural soda ash); the Solvay process (which uses sodium chloride and limestone); and the Hou process (which uses sodium chloride, ammonia, and limestone). Each of these three process routes employs different methods for producing an aqueous solution of sodium carbonate. The Solvay and Hou processes produce sodium bicarbonate (NaHCO3) by reaction of carbon dioxide (CO2) with an ammoniated brine solution.NH3+NaCl+CO2+H2O→NaHCO3+NH4ClThe sodium bicarbonate is then calcined to produce soda ash.2NaHCO3(s)→Na2CO3(s)+H2O+CO2 
The Solvay process produces a calcium carbonate (CaCO3) waste stream. The Hou process produces ammonium chloride (NH4Cl) as a by-product.
In the trona ore process shown below, mined trona ore (sodium sesquicarbonate (2NaHCO3.Na2CO3.2H2O)) is calcined to crude soda ash, which is then dissolved in water to remove insoluble minerals. Sodium carbonate monohydrate is then crystallized by evaporative crystallization. The isolated crystalline sodium carbonate monohydrate is then dried by air heating to produce anhydrous soda ash.2NaHCO3·Na2CO3·2H2O(s)→3Na2CO3+CO2+5H2ONa2CO3+H2O→Na2CO3·H2O(s) Na2CO3·H2O(s)→Na2CO3(s)+H2O
The trona ore process is the preferred process route to produce soda ash due to its lower raw material and energy costs relative to the Solvay and Hou processes. The trona ore process also produces less waste and byproducts than the other processes.
In addition to the trona ore mining process, soda ash is extracted from trona ore by solution mining. In solution mining, water is injected into the trona ore strata, and sodium carbonate and bicarbonate salts are dissolved into the brine solution. The brine solution is recovered from the trona strata and processed to recover soda ash values. Dissolved sodium bicarbonate in the brine is converted to sodium carbonate by steam stripping carbon dioxide gas. Similar to the trona ore process, sodium carbonate monohydrate is crystallized from the stripped solution by evaporative crystallization, and the resulting crystalline sodium carbonate monohydrate is air dried to produce anhydrous soda ash.NaHCO3·Na2CO3·2H2O(s)→NaHCO3+Na2CO3+2H2O2NaHCO3→Na2CO3+CO2+H2ONa2CO3+H2O →Na2CO3·H2O(s) Na2CO3·H2O(s)→Na2CO3(s)+H2O
Solution mining offers lower raw material costs than trona ore mining by the avoidance of subsurface ore mining operations. However, solution mining is notably nonselective to soda ash minerals. As a result, any soluble salts such as, but not limited to, salts such as sodium chloride and sulfate that are present in the trona ore strata are co-dissolved into the solution mining brine. In the same manner as sodium sesquicarbonate, sodium chloride and sulfate can dissolve to their solubility limits in the brine. Consequently, the concentrations of these impurities fed to the soda ash plant from solution mining can be significantly higher than from the trona ore mining process, and the potential to undesirably form anhydrous sodium carbonate during the crystallization processes is correspondingly higher.
In an effort to overcome the higher impurity concentrations present in solution mining operations, the inventors made improvements in the design and operation of soda ash crystallization systems that were observed to result in the prevention of the undesirable anhydrous sodium carbonate fouling, to improve yield, and to reduce the volume of the purge stream. Additional and unexpected benefits included the increased production of the crystalline monohydrate product and a significant reduction in power consumption during the crystallization process.